Various polymer films have been used widely in the field of electronic and electrical industry. They are often soldered when used as electrical parts or devices. As electrical parts making use of a polymer film, capacitor films and flexible printed circuit boards may be mentioned by way of example. Incidentally, when soldering an electrical part, the temperature of fused solder reaches as high as about 260.degree. C. There are however not many polymer films which can withstand such a high-temperature processing. On the other hand, those having excellent solder heat resistance are costly and hence involve a practical problem as industrial materials from the economical viewpoint.
Flexible printed circuit boards (FPC) with electronic parts mounted thereon, such as IC, are required to have good service properties such as high heat resistance, moisture resistance, flex resistance, nonflammability, copper peeling resistance and the like. Most of these service properties are however dependent on the corresponding properties of the polymer film employed as a base material. The utility of polymer films is expected to expand further to such fields as requiring a high level of heat resistance, for example, insulating films for motors and transformers.
For example, polyimide films have a glass transition temperature of 350.degree. C. or higher and hence possess sufficient heat resistance. Polyimide resins however do not have any melting point and lack melt flowability, so that they cannot be formed into films by any conventional biaxial stretching process. Moreover, they are inferior in moisture resistance, adhesion properties, and heat sealability.
Similarly, poly(arylene sulfides) which may hereinafter be abbreviated as "PASs", such as poly(phenylene sulfide) which may hereinafter be abbreviated as "PPS", can hardly be regarded as resins having sufficient solder heat resistance.
As a heat-resistant resin having the highest glass transition point (Tg) among crystallizable melt-forming materials, polyetheretherketone which may hereinafter be abbreviated as "PEEK" has recently been developed. A great deal of work is now under way with a view toward achieving its practical use. It has however been considered difficult to stably obtain stretched films of excellent mechanical strength from PEEK contain on an industrial scale because PEEKs contain aromatic rings in its backbone, its molecular chain is stiff and its stretchability is thus poor. It has therefore been proposed to roll a PEEK resin film or sheet by pressure rolls while applying a backward tension (Japanese Patent Laid-Open Publication No. 137116/1982). It has also been proposed to thermally set a film obtained in the above manner, thereby providing a base film for perpendicular magnetization (Japanese Patent Laid-Open Publication No. 93625/1985). PEEKs have a high crystalline melting point (about 340.degree. C.) and a high viscosity. Therefore, it is generally necessary to conduct its melt extrusion at an elevated temperature of 370.degree.-420.degree. C. Moreover, PEEKs have a high viscosity and are accordingly subjected to high shear stress. PEEK resin articles molded under such conditions therefore contain heat deterioration products at high concentrations. In many instances, heat deterioration products may not develop too much problems in molded articles which do not require stretching. They however develop extremely serious problems from the practical viewpoint where stretching is applied as in biaxially-stretched films. The existence of such heat distortion products becomes a serious problem, especially, in capacitor films and base films for FPCs, because smoothness is required for such films. In addition, PEEKs use 4,4'-difluorobenzophenone which is a relatively expensive raw material, and are hence costly. Use of these resins alone is therefore not advantageous from the economical standpoint.
On the other hand, polymer alloys formed by blending PEEK and PAS together have also been proposed (Japanese Patent Laid-Open Publication No. 172954/1982). Poly(phenylene sulfide) which has heretofore been used as a PAS has however been crosslinked (cured) for molding purposes, so that it cannot be formed into films. As a matter of fact, the above publication is silent about the formation of the polymer alloys into films. It has also been proposed to form a polymer alloy of PEEK and PAS into a sheet and to use it as a lining material or coating material for bearings (Japanese Patent Laid-Open Publication No. 60352/1983). However, it is not disclosed at all to form the polymer alloy into a biaxially-stretched film.
It has heretofore been difficult to conduct stable extrusion of a PAS/PEEK blend film, since the polymerization degree of a PAS is low and even if its blending with a PEEK having a high melting point is attempted, the melt viscosity of the PAS is so low at the melting point (about 380.degree. C.) of the PEEK that their mixing is poor and the melt extrusion of the resultant blend is unstable. Moreover, any attempt of biaxial stretching of the resulting film failed to achieve sufficiently stable biaxial stretching. No evaluation has therefore been made on biaxially-stretched films obtained in the above manner.